9

LTC1649

ages and expected loads may require different values.

Once the required value is known, the inductor core type

can be chosen based on peak current and efficiency

requirements. Peak current in the inductor will be equal to

the maximum output load current added to half the peak-

to- peak inductor ripple current. Ripple current is set by the

inductor value, the input and output voltage and the

operating frequency. If the efficiency is high and can be

considered approximately equal to 1, the ripple current is

approximately equal to:

∆I =

DC

DC =

L = inductor value

Solving this equation with our typical 3.3V to 2.5V appli-

cation, we get:

(0.8)(0.76)

(200kHz)(2

µH)

Peak inductor current at 10A load:

= 10.8A

10A +

1.5A

2

The inductor core must be adequate to withstand this peak

current without saturating, and the copper resistance in

the winding should be kept as low as possible to minimize

resistive power loss. Note that the current may rise above

this maximum level in circuits under current limit or under

fault conditions in unlimited circuits; the inductor should

be sized to withstand this additional current.

Input and Output Capacitors

A typical LTC1649 design puts significant demands on

both the input and output capacitors. Under normal steady

load operation, a buck converter like the LTC1649 draws

square waves of current from the input supply at the

switching frequency, with the peak value equal to the

output current and the minimum value near zero. Most of

this current must come from the input bypass capacitor,

since few raw supplies can provide the current slew rate to

feed such a load directly. The resulting RMS current flow

in the input capacitor will heat it up, causing premature

capacitor failure in extreme cases. Maximum RMS current

occurs with 50% PWM duty cycle, giving an RMS current

adequate ripple current rating must be used to ensure

reliable operation. Note that capacitor manufacturers’

ripple current ratings are often based on only 2000 hours

(3 months) lifetime; further derating of the input capacitor

ripple current beyond the manufacturer’s specification is

recommended to extend the useful life of the circuit.

The output capacitor in a buck converter sees much less

ripple current under steady-state conditions than the input

capacitor. Peak-to-peak current is equal to that in the

inductor, usually a fraction of the total load current. Output

capacitor duty places a premium not on power dissipation

but on low ESR. During an output load transient, the

output capacitor must supply all of the additional load

current demanded by the load until the LTC1649 can

adjust the inductor current to the new value. ESR in the

output capacitor results in a step in the output voltage

equal to the ESR value multiplied by the change in load

current. A 5A load step with a 0.05

Ω ESR output capacitor

will result in a 250mV output voltage shift; this is a 10%

output voltage shift for a 2.5V supply! Because of the

strong relationship between output capacitor ESR and

output load transient response, the output capacitor is

usually chosen for ESR, not for capacitance value; a

capacitor with suitable ESR will usually have a larger

capacitance value than is needed to control steady-state

output ripple.

Electrolytic capacitors rated for use in switching power

supplies with specified ripple current ratings and ESR can

be used effectively in LTC1649 applications. OS-CON

electrolytic capacitors from Sanyo give excellent perfor-

mance and have a very high performance/size ratio for an

electrolytic capacitor. Surface mount applications can use

either electrolytic or dry tantalum capacitors. Tantalum

capacitors must be surge tested and specified for use in

switching power supplies; low cost, generic tantalums are

known to have very short lives followed by explosive

deaths in switching power supply applications. AVX TPS

series surface mount devices are popular tantalum capaci-